One goal of automotive vehicle designers is an on-board diagnostic system that can measure the effectiveness of vehicle emissions control devices such as catalytic converters. In general a catalytic converter contains a catalyst in the exhaust path that causes certain target species of exhaust gasses to convert to different species that are thought acceptable for tail pipe emissions. For example, the catalytic converter removes HC, CO and NOx from the exhaust gas.
Over time or during certain engine conditions, catalysts may age and lose efficiency at which time the catalytic converter fails to convert the undesirable species of exhaust gases to the desired levels, impairing the vehicle's ability to meet tail pipe emissions standards or goals.